About Nikita Salnikov Tarnovski

G1 vs CMS vs Parallel GC

This post is following up the experiment we ran exactly a year ago comparing the performance of different GC algorithms in real-life settings. We took the same experiment, expanded the tests to contain the G1 garbage collector and ran the tests on different platform. This year our tests were run with the following Garbage Collectors:

-XX:+UseParallelOldGC

-XX:+UseConcMarkSweepGC

-XX:+UseG1GC

Description of the environment

The experiment was ran on out-of-the-box JIRA configuration. The motivation for the test run was loud and clear – Minecraft, Dalvik-based Angry Bird and Eclipse asides, JIRA should be one of the most popular Java applications out there. And opposed to the alternatives it is a more typical representative of what most of us are dealing with on the everyday business – after all Java is still by far most used in server side Java EE apps.

What also affected our decision was – the engineers from Atlassian ship nicely packaged load tests along the JIRA download. So we had a benchmark to use for our configuration.

We carefully unzipped our fresh JIRA 6.1 download and installed it on a Mac OS X Mavericks. And ran the bundled tests without changing anything in the default memory settings. The Atlassian team had been kind enough to set them for us:

-Xms256m -Xmx768m -XX:MaxPermSize=256m

The tests used JIRA functionality in different common ways – creating tasks, assigning tasks, resolving tasks, searching and discovering tasks, etc. Total runtime for the test was 30 minutes.

We ran the test using three different garbage collection algorithms – Parallel, CMS and G1 were used in our case. Each test started with a fresh JVM boot, followed by prepopulating the storage to the exactly the same state. Only after the preparations we launched the load generation.

Results

During each run we have collected GC logs using -XX:+PrintGCTimeStamps -Xloggc:/tmp/gc.log -XX:+PrintGCDetails and analyzed this statistics with the help of GCViewer

The results can be aggregated as follows. Note that all measurements are in milliseconds:

Parallel

CMS

G1

Total GC pauses

20 930

18 870

62 000

Max GC pause

721

64

50

Interpretation and results

First stop – Parallel GC (-XX:+UseParallelOldGC). Out of the 30 minutes the tests took to complete, we spent close to 21 seconds in GC pauses with the parallel collector. And the longest pause took 721 milliseconds. So let us take this as the baseline: GC cycles reduced the throughput by 1.1% of the total runtime. And the worst-case latency was 721ms.

Next contestant: CMS (-XX:+UseConcMarkSweepGC). Again, 30 minutes of tests out of which we lost a bit less than 19 seconds to GC. Throughput-wise this is roughly in the same neighbourhood as the parallel mode. Latency on the other hand has been improved significantly – the worst-case latency is reduced more than 10 times! We are now facing just 64ms as the maximum pause time from the GC.

Last experiment used the newest and shiniest GC algorithm available – G1 (-XX:+UseG1GC). The very same tests were run and throughput-wise we saw results suffering severely. This time our application spent more than a minute waiting for the GC to complete. Comparing this to the just 1% of the overhead with CMS, we are now facing close to 3.5% effect on the throughput. But if you really do not care about throughput and want to squeeze out the last bit from the latency then – we have improved around 20% comparing to the already-good CMS – using G1 saw the longest GC pause only taking 50ms.

Conclusion

As always, trying to summarize such an experiment into a single conclusion is dangerous. So if you have time and required skills – definitely go ahead and measure your own environment instead of adopting to one-size-fits-all solution.

But if I would dare to make such a conclusion, I would say that CMS is still the best “default” option to go with. G1 throughput is still so much worse that the improved latency is usually not worth it.

oncurrent-Mark-Sweep collector is most popular garbage collector of Java. CMS collector is popular for its better throughput and less pause time. Because for many applications, end-to-end throughput is not as important as fast response time. For example gaming applications need fast response time to make their gaming experience better, if any game hang for a second only, it lost its charm. As you know young generation collections do not typically cause long pauses, because of its small size and less amount of live objects survived. However, old generation collections is uncertain, can impose long pauses, especially when large heaps are involved. To address this issue, the Java HotSpot JVM includes a collector called the concurrent-mark-sweep (CMS) collector, also known as the low-latency collector

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